1. Field of the Invention
This invention relates generally to detectors with tunable spectral responses, including for example infrared (IR) detectors.
2. Description of the Related Art
For many applications in infrared sensing, it would be desirable to use a focal plane array (hereinafter referred to as “FPA”) with a tunable spectral response that can be tuned to aid in the detection of a particular type of target or the classification of a target to one of known classes of targets. The spectral response ideally could be optimized for use under particular imaging or detection/classification scenarios or to generate multispectral or hyperspectral images, which have a spectral radiance sampled at many wavelengths at each pixel in a scene. Typically, spectral tunability is obtained using an optical or electro-optical technique.
For example, one approach to spectral tunability uses multiple and/or tunable wavelength filters located upstream of the detector elements. This approach typically is capable of implementing between approximately 3 to 15 spectral bands. Images for the different spectral bands can be collected sequentially in time, by using a single detector element (or focal plane) and switching (or spectrally tuning) the wavelength filters as a function of time. Alternately, the different spectral bands can be collected simultaneously in time by using multiple detector elements, each utilizing a different wavelength filter. However, a mechanical switching or tuning system limits the frame-rate of the device and can be cumbersome and prone to mechanical failure. On the other hand, the use of multiple focal planes typically requires complex optical systems, which also lead to bulky and expensive solutions that are limited in the total number of available spectral bands.
Because of these limitations, hyperspectral sensors with more than 100 spectral bands typically use some sort of a shearing optic, such as a grating or prism, to separate the light incident on the sensor into either spectral or interferometric paths. A first spatial dimension of the FPA is typically used to collect the spectral data and a second spatial dimension is sued to collect a line image. The second spatial dimension is obtained through scanning. Other strategies instantaneously collect all of the spectral data by sacrificing spatial resolution through the subdivision of the array.
In another approach, the detector array consists essentially of two or three separate detector arrays stacked on top of each other, with each detector array designed to be sensitive to a different spectral band. For example, one array may be sensitive to the mid-wave infrared band and another array may be sensitive to the long-wave infrared band. These types of FPAs are typically referred to as two- or three-color cameras. The different detectors are electronically activated so that the FPA can switch between the two different wavelength bands. However, these cameras are limited to a very small number of spectral bands, typically two or three, and require a complex detector structure and read-out electronics just to achieve that. Furthermore, they are typically limited to switching between the two spectral bands, which are fixed in spectral response. They typically cannot implement continuous or fine-tuning of the spectral response. For example, the spectral response typically cannot be tuned to an arbitrary center wavelength and spectral width.
Among non-tunable IR detectors, quantum dot infrared photodetectors (QDIPs) have shown steady progress in their performance ever since their first demonstration. The design, performance characteristics, and limitations of prior art QDIPs, such as dots-in-well (DWELL) detectors, are reviewed by Raghavan et al., in Applied Physics Letters Vol. 81 Number 8, Aug. 19, 2002, which is hereby incorporated by reference. QDIPs have demonstrated normal incidence operation in the mid-wave infrared (MWIR, between approximately 3 and 5 μm), the ling wave infrared (LWIR, between approximately 8-12 μm) and in the very ling wave infrared (VLWIR, for wavelengths longer than approximately 14 μm). For certain forward-looking infrared applications, such a broadband response may be desirable. For multispectral/hyperspectral imaging applications, however, it is not.
Thus, there is a need for semiconductor detectors with tunable spectral responses, especially responses that can be tuned by electrical signals, including for use in the IR, typically 2-30 μm. There is also a need for detectors with tunable and narrow spectral bands suitable for use with multispectral/hyperspectral imaging.